Weird span for `#[bench]` method error

[estebank]

Given

#[bench]
fn bar(x: isize) { }

The current output is suboptimal:

error[E0308]: mismatched types
  --> $DIR/issue-12997-2.rs:16:1
   |
LL | fn bar(x: isize) { }
   | ^^^^^^^^^^^^^^^^^^^^ expected isize, found mutable reference
   |
   = note: expected type `isize`
found type `&mut __test::test::Bencher`

[euclio]

Unfortunately, I'm not sure that it's possible to get a better error message without making #[bench] a full-blown compiler plugin. Since #[bench] expansion happens before resolution, we can only check that the argument list has the expected number of arguments. We can't check the name of the argument either, because it could be aliased.

rust/src/libsyntax_ext/test.rs

Lines 327 to 329 in 354a29a

	// NB: inadequate check, but we're running
	// well before resolve, can't get too deep.
	decl.inputs.len() == 1

So, we get this error after the macro has been expanded and the types fail to match. The reason #[test] can get better diagnostics here is because it can just assert that there are no args. You'll get a similarly confusing message if you write a test with a signature like fn test() -> String.